Correct me if I'm wrong, but with the exception of fonts... Sigil will automatically include the rest of that stuff (styles, images) if it's correctly linked in the html (and the style and image files are in the correct path relative to the location of the html) file being passed to Sigil as an argument.
I just tested:
"C:\Program Files\Sigil\sigil.exe" test.xhtml
... and when Sigil opened, it had pulled in any and all css files and images that were referenced in test.xhtml.
Or were you referring to something else?
EDIT: Now that you mention it, though... it
would be sorta cool if you could feed Sigil an OPF file and then it would be able to pull all the rest of the files together from that. Mind you; I'm not trying to create work for anyone. Just wish-listing a bit.